1. Field of the Invention
The present invention relates to an automatic speed control system for a vehicle, and more specifically to a speed control system which can find the termination of an uphill road.
2. Description of the Prior Art
There has been known an automatic speed control system in which a throttle opening is controlled based on a deviation of an actual vehicle speed from a target speed to get the target vehicle speed. Such a system is disclosed in Japanese Patent Public Disclosure Application (JP A)No. 61-285233, laid open to the public in 1988 as Japanese Patent Public Disclosure No. 63-137,037.
In this automatic speed control, it is proposed that when the vehicle runs into an uphill road when the automatic speed control is being carried out, a transmission makes a down shift operation, such as one from a fourth stage, to a third stage so as to increase a drive torque to maintain a target speed during running on the uphill road. When the vehicle reaches the end of the uphill road, the transmission immediately makes an up-shift operation, such as one from the third stage, to the fourth stage so as to save fuel.
In order to make the above control, it is necessary to detect the end of the uphill road. If the uphill road has a constant gradient, the throttle opening is remarkably reduced when the vehicle stops running uphill and starts to run on a flat road. Therefore, the end of the uphill road can be detected by detecting the remarkable reduction of the throttle opening. In view of this, it has been proposed to determine that the end of the uphill road has been reached when the throttle opening is reduced beyond a predetermined value.
This manner for detecting the end of the uphill road can be properly applied for a long uphill road during which the throttle opening settles to a substantially constant value after a down-shift operation.
It should, however be noted that in some cases, an uphill road may terminate before the throttle opening settles down. Therefore, the above manner for detecting the uphill road can not be applied for a relatively short uphill road which ends before the throttle opening settles down after the down-shift operation.